fix(search): report failed searches instead of false misses

[jonathanKingston]

A ripgrep or grep error currently becomes “No matches found.” Invalid regexes and unreadable search roots can therefore make the agent conclude relevant code does not exist.

Propagate unsuccessful command exits with the diagnostic message, while preserving exit code 1 as a valid empty search. Regression coverage distinguishes a regex failure from a legitimate miss.

Validation: pnpm run check. The error regression fails on current main. Uses the existing tool-error presentation; no DOM or layout changes.

[github-actions]

🖥️ PR preview

• Marketing site
• Browser demo
  Interactive, with a mocked backend — jump to a scenario:
• Builds a cupcake site
• Agent-proposed edits open the Changes panel
• Markdown list indentation
• Responsive composer footer
• Subagent display visual reference
• Subagent stays ungrouped beside reading tools
• Semantic search subagent markdown
• Light-theme approval with a bright accent
• Light-theme syntax, fills, and selection
• Grouped outside-sandbox command approval
• Remote desktop discovered-port list with one selected
• Settings scroll + sticky footer geometry
• Per-model reasoning / temperature / top-p
• Thread switch hydration notice
• Thread switch hydration failure
• Chat layout styling
  _{Static demo of the renderer against a mock API — the browser pane, terminal, and all main-process IPC are stubbed, so it never proves main-process behaviour. Rebuilt whenever a push changes the demo bundle; these links go live once the queued deploy lands, shortly after this comment. Removed when the PR closes.}